Carrier telegraph system



Oct. 27, 1953 H. T. PRIOR' A Mar/ Mark AM Space iS aace igaace InbentorHECTOR T. PR 10R B wa ' Attorney Patented Oct. 27,' 1953 UNITED STATESPATENT l 2 r ogmnmn TELEGRAPH sesame fficton'ihoina's fi'ioi', London,England, assigiiiir to. International Standard v.Ele'ctric Corpoi'fienon, New York, N. Y., a corporation or Delaware Aiiiiiidtioii septlenibr18, 1951, Serial fi 217,665 In Great Britain September 22, 1950 7Clailiis.

This invention relates to receivers for amplitude modulated amertelegrap s stems. By the term amplitude riiodulatd carrier telegraphsystems is meant systems in which the transnutter transmits constantfrequency carrier waves, amplitude modulated by telegraph signals, overva line orotlier' communication medium to a receiver incorporating meansfor extracting the telegraph signals from the modulated carrier waves. pc

In the usual aniplitiide modulation telegraph systems, it is well Knownthat as a result of the transmission characteristics of the transmissionpath, the substantially rectangular signal waves originally obtainedfrom the transmittingteleprinter or other signalling device, appear atthe receiver after eemoduiauqn in considerably rounded forin. Thfeproblem therefore arises of deriving from the received signal wave thecorrect instants of transition froni'iriark to space and vice-versa; I Vp The 'inethods of'de-modulating' the received signal which have ingeneral been I adopted hitherto are based upon the principle that, pro-Jvided the signalling spee does not ez'icee'd a certairi max mum value,the internal transmissiqn characteristics of the 'rec'eifvingfcir'ouitcan be so chosen that the times when the received signal amplitudepasses through a value, equal to half the maximum amplitude arespacedsubstantially in the same way as the vertical edges of the rectangunrsignals drigmauyi transmitted, If a bias equal to half the ma imumamplitude and of opposite polarity is applied to such a signal wave, thesignal instants can readily be maintained correctly. since the general,amplitude level of the tfelnsiriittedsig nals ma v ry during a period ofsignalling asa fres'ult for example, due to the vagaries of thetransmitting medium, it is usual to derive this bias froin the receivedsignal waveaseir. l As thetransmitting "speedisnincreased, there comes apoint at which the transitions defining the shortest signals, interferewith'each other and prevent the full amplitude bein'g attained; other'w'drd, a transition rrdiii say, marlgto space .may not be completedbefore the succeeding transition frorn space to mark beginsand the twowave-form transientsinterseet before the point of maximum amplitude isatginea Provided the tinie interval betweer'i'tr'ansitions is not made toosmall, the reduction in amplitude of these shorter waveSdQesj not affectthe height .oii'the transition eurqesatwhi h the po nts l "responding tothe-"angina signal transitions occur. Thus; application of the samevalue pf ias as before will sun give the correct result, if, however,thisbias were simplyarrangedtofollow the contour of thejreceived signal;the dim is ed 5 signals would have applied to them a dimifiis ed biasvoltage.

To overcome this, difiicujlty ary to introduce into th'eb amplitude isreduced te inpor v I shorter signals. v A receiving arran carriertelegraph system in wh' is employed is disclosed in th I I V theco-pending application of J; T r'y; Hargreaves, and H. T. Piio'rgfiled1949, and bearing SerialNo'; 1301326, No. 2,613,272, This met odslifiersfm advantages that on the one hand; a in the signal amplitudefduefor e change in the transmitting jcond' s appreciated until a number of"cha cters been incorrectly biasse'd 'an'd lost. on t i. other hand, theoccurrence oia short'bur'st' of ghl e1 interference results in'anartificial 4 bias potential so that characters are ag v until the biasfalls back to the normal maximum signal amplitude; I a 4 1 Accordingtothe' present inventioi'i 'th I vided a receiving circuit for ramplitue In lated carrier telegraphsystem comprising ne for deriving ab'ilss'ing' potential from the rec signals to be combined therewith in dmaintain in the combined signal co stants of transition between markcharacterised by the provision of; riving two separatesets' of su' ch btials, one for the transitions fr or a negative potential islpresentdepenchn whether single-current or double-current "signalling is in'use.V V

T n i n illn whe'inqre 1 1 3 1 with reference to the accompanyingdrawings in 'Fig. 1 comprises a set o V which will be used in the explinvention and,

Fig. 2 is a circuit diaEiain of pa i ttf afteiegraph receiverincorporating an embodiment of the invention.

In Fig. 1 curve A shows the rectangular signals produced by thetelegraph transmitter. These are generally in double-current form, thatis to say they are in the form of equal positive and negative voltagesor currents with respect to a zero axis shown in dotted line. Thepositive signals will be designated marks and the negative signalsspaces. The signal waveform shown in curve A may be regarded as beingmade up of a combination of eight equi-length elements namely S M M S SS M S. Furthermore the first seven elements will be seen to constitute aconventional start-stop teleprinter signal combination containing astart (space) element, five permutable elements which in this caserepresent the character A, and a concluding stop (mark) element. As iswell known two or more consecutive elements of the same kind aretransmitted without a break so as to constitute a single marking orspacing impulse. In the following description a single mark (or space)element will be referred to as a short mark (or space) signal while twoor more consecutive mark (or space) elements will be referred to as along mark (or space) signal.

When the rectangular signals of curve A are applied to modulate acarrier wave, the arrangement is such that a carrier wave of constantamplitude is transmitted for mark signals and no carrier wave for spacesignals, so that transmission is efiectively of single-current type.

When the modulated carrier wave is rectified at the receiving end, therecovered signal wave is also of single-current type and may appear asshown in curve B. Owing to the transmission characteristics of thetransmission path, the rectangular signals are considerably rounded andthe leading and trailing edges are appreciably inclined from thevertical as indicated in curve B. If the marking signals are long enoughas shown at LM in curve A, the curve has time to reach the correctmaximum amplitude V as shown at LM in curve B. Similarly, if the spacingsignals are long enough, the curve has time to reach the zero axis 0.However, with a short marking signal such as SM of curve A, the signaltransits from space to mark and mark to space interfere with each otherand produce an attenuated wave form as shown at SM of curve B. In bothcases, however, the points on the curve corresponding to the originalsignal transits occur at a level of V/Z which represents half themaximum amplitude. It follows that if a biassing wave opposite in signbut similar in shape to the signal wave of curve B be derived, thepoints at which this wave passes through a value of V/2 will be spacedapart by the same distances as those which separate the half-amplitudepoints on the curve B.

Turning to the curves at C of Fig. 1 there are shown two biassin waves,one (0!) for dealing with space to mark transitions and. the other (c2)for mark to space transitions. Each of them is of the same shape as thereceived signal of curve B but reversed in sign and diminished inamplitude. Curve cl is advanced in phase relative to the signal wave andcurve c2 is delayed with respect to the signal wave by the sameinterval. This interval is chosen to make the minimum negative peaks Xand Y occur at the signal transits in the case of the shortest marksignal S M. The amplitude of the bias waves is such that they assumehalf the steady-state value V/2 oi the signal wave at the signaltransits of a long mark signal LM These two waves are applied to thereceived signal wave in the form of a combined bias wave whose value atany point is equal to that of the stronger wave at that point. Thecombined bias wave is shown as a dotted curve 03.

The wave produced by combining the bias wave c3 with the received signalwave is shown in curve D of Fig. 1. It will be seen that this wavepasses through zero at times corresponding to the instants of transitionof the original rectangular telegraph signals of curve A.

A method of producing the bias wave 03 and of adding it to the receivedsignal wave B of Fig. 1 will now be explained with reference to Fig. 2.Referring now to Fig. 2, teleprinter signals after demodulation by aprevious stage (not shown) of the receiver appear across terminals TIand T2 with a waveform such as that shown by the curve B of Fig. 1.During a mark signal the potential of TI rises to a value V relative toT2 which is assumed to remain at zero potential. During a space signalboth terminals are at zero potential. The demodulated wave is applied totwo delay networks in series, LI, CI and L2, C2. These networks are ofthe type in which a voltage applied at the input terminals appears atthe output terminals altered in phase but not in magnitude. The two biaswaves cl and 02 (Fig. l) are derived from potentiometers RI and R2respectively. If the output signal wave is taken from terminals T3 andT4 between the two networks it will be evident that it will be delayedrelative to the space to mark bias cl and advanced relative to the markto space bias 02. By this means the space to mark bias cl appears tolead the signal wave although this is in fact impossible since the biaswave cannot appear until the signal wave appears.

A portion of the signal Wave appearing across potentiometer RI isapplied via rectifier X! to one electrode of a condenser C3. Similarly aportion of the signal wave appearing across potentiometer R2 is appliedvia rectifier X2 to the same electrode of condenser C3. Thetime-constant of C3 and its shunt resistor R3 is so chosen that thecondenser remains charged up to the value of the stronger bias wave atany instant, gluslproducing the combined bias wave 03 of It will benoticed that in the middle of a lon spacing period the combined signaland bias wave of curve D goes back to zero and there would be sometendency to instability in a relay which was being supplied fromterminals T3 and T4. Furthermore, during a prolonged period of space duefor example to the opening of the line there would be a possibility ofthe receiving relay being changed over to mark by impulsiveinterference. In order to meet these contingencies an auxiliary biassingpotential shown purely diagrammatically as a single cell BI is connectedthrough a rectifier X3 to terminal T4. This puts a spacing bias on theoutput circuit in the absence of any other potential. When the voltageacross C3 rises to a value above that of Bi, X3 is blocked and BI doesnot have any effect.

The circuit of Fig. 2 can be simplified (with little change inperformance) by omitting the second delay network L2, C2. With thismodification the mark to space bias wave 02 follows the signal wavewithout delay and the condenser C3 plays a vital part in producing themark to 5 space bias, the time constant of R3, C3 being suitablyadjusted to obtain a close approximation to the correct biassingpotential at the signal instants.

While the principles of the invention have been described above inconnection with specific embodiments and particular modificationsthereof, it is to be clearly understood that this description is made byway of example and not as a limitation on the scope of the invention.

What is claimed is:

1. A receiving circuit for an amplitude modulated carrier telegraphsystem comprising means for deriving a first biassing potential wavefrom the received signal wave, means for deriving a second biassingpotential wave from the received signal wave, means to advance the phaseof said first biassing wave with respect to the phase of the receivedsignal wave, means to retard the phase of said second biassing wave withrespect to the phase of the received signal wave, and means coupled tosaid biassing potential wave deriving means for applying to saidreceived signal wave whichever of said biassing waves is of greateramplitude at any given instant.

2. A receiving circuit as claimed in claim 1, wherein said means forderiving a first biassing potential wave comprises a, first delaynetwork, means for applying the unbiassed signal wave to the input tosaid first network, means for deriving from the input to said firstnetwork the space to mark biassing wave, means for deriving from theoutput of said first network mark to space biassing wave and means forderiving from the output of said first network-the biassed signal wave.

3. A receiving circuit as claimed in claim 2, wherein said means forderiving a second biassing wave comprises a second delay network coupledto the output of said first network.

4. A receiving circuit as claimed in claim 2, further comprisingauxiliary biassing means coupled to said means for deriving both saidpotential waves, whereby a predetermined biassing potential is appliedto said receiving circuit independent of the receipt thereby of anyreceived signal waves.

5. A receiving circuit as claimed in claim 1, wherein said means forapplying to said received signal wave the biassing wave of greatestamplitude comprises a time constant network connected in common to bothsaid biassing potential Wave deriving means.

6. A receiving circuit as claimed in claim 5, wherein said biassingpotential wave means comprises a plurality of unidirectional devicespoled in the same direction, each one coupled between a different one ofsaid means for deriving a biassing potential wave and said time constantnetwork.

7. A receiving circuit as claimed in claim 6, wherein one of saidunidirectional devices is coupled between said auxiliary biassing meansand said time constant network.

HECTOR THOMAS PRIOR.

References Cited in the file of this patent UNITED STATES PATENTS Number

